This application claims priority of European Patent Application EP01811218.5, the content of which is hereby incorporated.
The present invention concerns a measuring machine, notably a column for measuring longitudinal dimensions, for example a height-measuring column.
Height-measuring columns are described for example in document U.S. Pat. No. 4,924,598. They are used for example for measuring or comparing dimensions, for example in mechanical workshops. A measuring column generally comprises a fixed supporting frame with a base, a carriage that can be displaced vertically along the supporting frame, a device for driving the carriage and a system for measuring the carriage""s vertical position. A probe tip is connected to the carriage and is designed for being brought into contact with the piece to be measured. Some measuring columns comprise a base provided with means for creating an air-cushion in order to easily displace the height-measuring column on the work surface.
The measuring column described in U.S. Pat. No. 4,924,598 comprises an electric motor lodged in the base and driving the lower pulley through an axle or transmission belt. The lower pulley drives a driving belt coupled to the carriage, as well as a counterweight moving in opposite direction from the carriage. The driving belt is tensed between the lower and the upper pulleys. The carriage comprises wheels for pressing on the guiding rails attached to the supporting frame.
The electronic measuring system allows the position of the carriage, and thus of the probe tip, to be determined and displayed on an electronic display. The resolution and precision that is expected of this type of measuring columns is on the order of the micron.
This precision depends for an important part on the contact force between the probe tip and the piece to be measured. A substantial contact force causes a flexion of the probe tip and/or of the piece, or even an elastic deformation of the material, that can influence the measuring. The contact force between the probe tip and the piece to be measured must thus be minimal or, in any case, identical at each measuring.
It is thus essential to ensure that the traction force exerted by the cable or belt on the carriage should be reproducible whatever the carriage""s longitudinal position. For this purpose, it is necessary to design the column so that the driving of the carriage should be as soft and regular as possible. In particular, it is necessary to prevent as much as possible any shocks and jerks.
As previously mentioned, measuring columns are however often moved on an air-cushion during measuring. The carriage is displaced vertically by means of the motor. The probe tip comes into contact with the piece to be measured. The operator handles the control panel or the height-regulating wheel. The measuring column is thus subjected to different types of shocks and accelerations during a normal measuring session.
It has been observed within the framework of these inventions that these accelerations sometimes cause the counterweight to swing. The latter is suspended by the driving cable or belt and is thus free to move laterally. When the counterweight is at the bottom of the column, shocks can cause a swinging of the counterweight of relatively great amplitude and which dies only very slowly. This is in particular the case with modern measuring columns in which the tension of the cable or belt is weak so as to limit the forces and moments exerted on the measuring carriage. In this case, the driving belt exerts only a limited braking force on the swinging counterweight.
This swinging causes a variable traction on the driving belt or cable that is reflected on the carriage and the probe tip. The pressing force of the probe tip on the piece to be measured is influenced by this traction, which greatly disturbs the measuring.
Furthermore, substantial swinging of the counterweight can also occur during transportation of the measuring column and can possibly even damage it.
U.S. Pat. No. 4,399,617 describes a measuring column in which the counterweight is guided by a rod traversing it through a longitudinal opening in its middle. The counterweight does not move freely but slides around this rod. Any possibility of swinging is thus prevented. The friction between the counterweight and the central rod however creates an additional tension on the driving cables, which are reflected on the measuring carriage and generate additional constraints. If the rod is not perfectly vertical, the friction further depends on the counterweight""s vertical position, causing a pressing force that varies according to the carriage""s position. The rod and the opening through the counterweight must be manufactured with a high accuracy to ensure a constant friction on the entire course run by the counterweight; this requirement leads to a considerable cost increase.
It is an aim of the present invention to propose a column for measuring longitudinal dimensions that avoids the disadvantages of the prior art columns. In particular, it is an aim of the present invention to make a column for measuring longitudinal dimensions in which the disturbances caused by the counterweight are reduced to a minimum.
According to the invention, these aims are achieved by means of a measuring column comprising the characteristics of claim 1, preferred embodiments being furthermore indicated in the dependent claims.
In particular, these aims are achieved by means of a guiding surface for restricting the counterweight""s movements. A play is provided between the counterweight and the guiding surface; this play is sufficient to avoid any contact between the counterweight and any guiding surface when the counterweight is displaced vertically.
This has the advantage that the counterweight""s swinging can be greatly reduced without however creating an additional friction. Possible oscillations of the counterweight can be only of very limited amplitude and will thus die very quickly.
The selected play between the guiding surface and the counterweight is the result of a compromise. A play that would be too considerable is insufficient for preventing the disturbances caused by the counterweight""s swinging. A very restricted play entails close manufacturing tolerances to avoid any contact between the counterweight and the guiding surface. If the play is insufficient, even a very accurate manufacturing process will not prevent the risk of collisions when the measuring column is placed on a surface that is not absolutely horizontal.
Tests and trials have shown that the risk of collision between the counterweight and the guiding surface increases rapidly when the play is less than 0.2 millimeters. On the other hand, a play less than 5 millimeters is generally sufficient, in the case of medium-height measuring columns, for the time required for the counterweight""s swinging to die to be comparable to the time required for performing the measurement and stabilizing the carriage. The optimal value of the play is thus comprised between 0.2 and 5 millimeters, preferably between 0.5 and 2 millimeters. Conclusive trials have notably been effected using a play of 1 millimeter.